The skeletal-muscular system of the human body consists of the 206 bones and over 650 muscles that maintain the skeletal structure, protect and support the internal organs, and help the body move. During recent years a great deal of interest has been shown in the exercise of important body muscles for the development of specific strengths and physical abilities and for the development of desired body shapes and proportions. Particular attention has been devoted to the development of the chest muscles (pectoralis major), back muscles (trapezius), upper arm muscles (biceps and triceps) and principal upper leg muscles (quadriceps femoris) through weight lift and force application and resistance exercise programs and machines. Amateur and professional athletes and body builders, both male and female alike, spend many hours per week in such exercise programs utilizing a broad range of apparatus from simple barbells to complex and sophisticated exercise machines. Although many of the body building exercise programs include, to some degree, development of shoulder muscles, no specific machine has heretofore been designed to provide a total exercise system for shoulder muscle development.
The human shoulder consists of two bones: (1) a broad, flat shoulder blade (or scapula) in the back; and (2) a slender collar bone (or clavical) in front. The large bone of the upper arm (the humerous) has a round head-like portion that fits into a shallow depression of the scapula, to form a ball-and-socket joint, which allows great freedom of movement. Seventeen muscles serve to move and cover the joint, including (principally) the deltoid muscle, upper portions of the pectoralis major, biceps and triceps muscles, and lower portion of the trapezius muscle. The shoulder joint, because of its great mobility, is the most frequently dislocated joint of the human body. The clavicle bone, lying very close to the body surface, is one of the bones most frequently fractured during athletic activities.